The experiment at LHC

Stefano Lami

INFN Pisa

on behalf of the TOTEM Collaboration

Total cross-section

Elastic Scattering

jet

jetb

Diffraction: soft and hard

b

Forward physics

TOTEM Physics OverviewUltimately

~1% precision

Over a wide t range

2/21

Predictions at 14 TeV: 90-130 mb

• Available data not decisive

• Aim of TOTEM: ~1% accuracy

mb 1.41.2

2.1 5.111 totLHC (14TeV):

COMPETE Collaboration fits all available hadronic data and predicts:

[PRL 89 201801 (2002)]

Dispersion relation fit

Best fit TOTpp rises as

~ ln2 s

Physics Motivation (1)

3/21

It will distinguish between several models

Elastic scattering (diffraction in general)• theoretical understanding not complete• number of approaches: Regge, geometrical, eikonal,QCD, . . . rather ⇒incompatible predictions• intimately related to the structure of proton

Big uncertainties at large |t|: Models differ by ~ 3 orders of magnitude!

3-gluon exchange at large |t|:

8~d

tdt

Islam Model:

b* = 90 m

b* = 2 m

b* = 1540 m

|t|

Physics Motivation (2) …

4/21

… and moreTOTEM will measure the 10-3< | t | ~(pq)2 <10 GeV2 range with good statistics

p-p Interactions

Incident hadrons acquire colour and break apart

Incident hadrons retain their quantum numbers remaining colourless

Non-diffractive Diffractive

Colour exchange Colourless exchange with vacuum quantum numbers

GOAL: understand the QCD nature of the diffractive exchange

dN / d = exp (-) dN / d = const

5/21

= -ln tg /2x= Dp/p

= -ln

p. 6Indicative cross-sections at 14 TeV

~30 mb

~10 mb

~7 mb

~1 mb

M

MDouble Pomeron Exchange

Double Diffraction

Single Diffraction

Elastic Scattering

~65 mb ~ 58 %

<< 1 mb

Dominant Event Classes in p-p Collisions

~ 42 %

Processes with large cross-sections !

London Workshop 30.03.2009 6/21

( = -ln tg /2)f

h

Stefano Lami 7/21

TOTEM Experimental Apparatus• Physics requirements: forward proton detectors and large pseudorapidity coverage Roman Pots & tracking telescopes T1 and T2 (inelastic evts + Vtx reco)• all detectors symmetrically on both sides of IP5• all detectors L1 trigger capable

T1: 3.1 < |h| < 4.7

T2: 5.3 < |h| < 6.5

Stefano LamiLondon Workshop 30.03.2009 8/21

TOTEM Coverage

Cou

rtes

y by

Jan

Kas

par

9/21

TOTEM Roman Pots

HALF OF THE RP STATION

Horizontal RP Vertical RP BPM

Beam

Thin window 150 mm

10 silicon detector planes

pitch 66mm

Overlap for alignment

Protons at few rad angles w. RPs at 10 + d from beam

`Edgeless’ detectors to minimize d (sbeam ~ 80 mm at RP220 w. b*=1540m)

Each RP station has 2 units, 4m apartEach unit has 2 vertical insertions (‘pots’) + 1 horizontal

Ongoing Detector Installation:

RP220 (147)

fully (partially) equipped by June

Stefano LamiLondon Workshop 30.03.2009 10/21

T1 Telescope

3m

1 station per side of IP5• each station: 5 planes• each plane: 6 trapezoidal CSC detectors• L1 trigger capability• Cathode Strip Chamber detector– 3 coordinates: 2 cathode strips, 1 anode wire– Resolution ≈ 1 mm• Primary Vtx reconstruction to discriminate background (not beam-beam events) for the measurement of Ninel

• Multiplicity measurement

¼ of T1

Installation foreseen for May/June:

One arm eventually in September

Stefano LamiLondon Workshop 30.03.2009 11/21

T2 Telescope

1 station on each side of IP5• each station: two halves (left/right)• each half: 10 (5×2 back-to-back mounted) GEM detectors• L1 trigger capability• Triple Gas Electron Multiplier detectors– double readout: strips (radial) and pads (coarse radial and azimuthal)– resolution: radial 100 mm, azimuthal 0.8◦

• Vtx reco. + Multiplicity measurement

Ongoing installation:

fully done by May

12/21

stot(~1%), low t elastic, stot(~5%), low t elastic, high t elastic, soft diffraction (semi)-hard diffraction hard diffractioncross section

luminosity mb b nb

* (m) 1540 90 2 0.5

L (cm2 s1) 1028 1030 1031

1033

TOTEM runs Standard runs

TOTEM Physics and LHC OpticsFeasible physics depends on running scenarios:• luminosity• beam optics (β*)

Þ acceptance of proton detectorsÞ precision in the measurement of scattering angle / beam divergence */1

EARLY

London Workshop 30.03.2009 Stefano Lami

Optical functions: - L (effective length); - v (magnification);

- D (machine dispersion)

Describe the explicit path of particles through the magnetic elements as a function of the particle parameters at IP.

Define t and range (Acceptance)

= p/p; t = tx + ty; ti ~ -(pi*)2

(x*, y*): vertex position at IP

(x*,y

*): emission angle at IP

Proton transport equations:

x = Lxx* + vx x* + D

y = Lyy* + vy y *

ExampleSame sample of diffractive protons at different *

- low *: p detected by momentum loss ()- high *: p detected by trans. momentum (ty)

Details on Optics

13/21

Detector distance to the beam: 10+0.5mm

-t = 0.01 GeV2

-t = 0.002 GeV2

Beam

* = 1540 m

b*=1540 m

b*=2 m

b*=90 m

log(-t / GeV2)

0.002 0.06 4.0

Acc

epta

nce

Det. dist. 1.3 mm 6 mm

b* = 1540 m L = 1028 – 2 x1029 95% of all p seen; all x

b* = 90 m L = 1029 – 3 x1030 65% of all p seen; all x

b* = 0.5– 2m L = 1030 – 1034 x > 0.02 seen; all t

b = 0.5m - 2m

x r

esol

ved

RP 220m

Optics properties at RP220:

Diffractive Proton Acceptance in (t, x):(contour lines at A = 10 %) RP220

Proton Acceptance

Elastic Scattering

90m: see all protons0.03 < |t| < 10 GeV2

Low b*: see protons0.02<x<0.2 or 2< |t| < 10 GeV2

22 4

2

2/ 2

2 2

2

d

dt

4

1

16

B ttot

tot B t

c G t

t

G te

t

ec

,22

0

16L

1elastic nuclear

tot

t

dN

dt

,L tot elastic nuclear inelasticN N

, 02

,

( / )16

1

elastic nuclear ttot

elastic nuclear inelastic

dN dt

N N

Total Cross-Section and Elastic Scattering at low |t|

TOTEM Approach: Measure the exp. slope B in the t-range 0.002 - 0.2 GeV2 , extrapolate d/dt to t=0,Measure total inelastic and elastic rates (all TOTEM detectors provide L1 triggers):

Related by the Optical Theorem:

Coulomb scattering

Nuclear scattering

Coulomb-Nuclear interference

a = fine structure constantf = relative Coulomb-nuclear phaseG(t) = nucleon em form factor = (1 + |t|/0.71)-2

r = Re/Im f(pp)

Measurement of the Inelastic Rate

s[mb]

trigger loss [mb]

systematic error after extrapolations [mb]

Non-diffractive inelastic

58 0.06 0.06

Single diffractive 14 3 0.6

Double diffractive 7 0.3 0.1

Double Pomeron 1 0.2 0.02

Total 80 3.6 0.8

Acceptancesingle diffraction

simulatedextrapolated

detectedLoss at low diffractive masses M

• Inelastic double arm trigger: robust against background, inefficient at small M• Inelastic single arm trigger: suffers from beam-gas + halo background, best efficiency• Inelastic triggers and proton (SD, DPE): cleanest trigger, proton inefficiency to be extrapolated• Trigger on non-colliding bunches to determine beam-gas + halo rates.• Vertex reconstruction with T1, T2 to suppress background• Extrapolation of diffractive cross-section to large 1/M2 assuming d /s dM2 ~ 1/M2

16/21

Combined Uncertainty in tot

b* = 90 m 1540 m

Extrapolation of elastic cross-section to t = 0: ± 4 % ± 0.2 % Total elastic rate (strongly correlated with extrapolation): ± 2 % ± 0.1 % Total inelastic rate: ± 1 % ± 0.8 %

(error dominated by Single Diffractive trigger losses) Error contribution from (1+r2)

using full COMPETE error band /dr r = 33 % ± 1.2 %

Total uncertainty in stot including correlations in the error propagation: b* = 90 m : ± 5 %, b* = 1540 m : ± (1 ÷ 2) % .

Slightly worse in L (~ total rate squared!) : ± 7 % (± 2 %).

Precise Measurement with b* = 1540 m requires:• improved knowledge of optical functions• alignment precision < 50 mm

20

1

/16 el t

elto

nt

i el

dN dt

N N

2

0

21

/16el inel

el t

N N

dN dt

L

17/21

Stefano LamiLondon Workshop 30.03.2009 18/21

Early Physics with TOTEM (Ep = 5 TeV & b* = 3m)

RP220 Acceptance:

- elastic scattering 1 < |t| < 12 GeV2

- diffractive protons 0.02 < x < 0.18- resolution: σ(ξ) <∼ 6 · 10−3

very similar to Ep = 7 TeV !!

*b =1540

*b =90

*b = 0.5 - 3

Log(M) (GeV)

DPE

Protons seen for > 2%

Very first measurements with low b* optics

Using horizontal RPs

dSD/dM (SD events with high mass) 0.02 < < 0.18 2 < M < 6 TeV

(M)/M = 24 %

dDPE/dM (DPE high mass) 250 < M < 2500 GeV

(M)/M = 2.13.5 %

Using vertical RPs: high t elastic scattering dElastic/dt 1 < |t| < 12 GeV2 (t) 0.2 |t|

pp p

MX2 = s

Rapidity Gap

-ln

pX

SD

MX2 = 1 2s

Rapidity Gap

-ln 2

Rapidity Gap

-ln 1

p1 p2X

2 pp21 p p1

DPE

19/21

MPI@LHC 2008 20/21

Acceptance:3.1 < < 4.7 (T1) & 5.3 < < 6.5 (T2)

() = 0.04 0.2, no mom. & info

minimum bias

dN

ch/d

h

[1/u

nit

]

Particellecaricheper evento

T1/T2: charged multiplicity● Measurement of charged multiplicity for different processes

● Important also for cosmic ray physics (e.g. for MC generator validation)

● Identification and measurement of rapidity gaps

single diffractive

TOTEM ready for LHC restart will run under all beam conditions will need high β∗ optics → will require β∗ = 90m optics for early running Early measurements

low *: - study of SD and DPE at high masses - elastic scattering at high |t| - measurement of forward charged multiplicity * = 90 m:

- first measurement of tot (and L ) with a precision of ~ 5% (~ 7%) - elastic scattering in a wide |t| range - inclusive studies of diffractive processes - measurement of forward charged multiplicity

Later Measurement of total pp cross-section (and L ) with a precision

of 1÷2 % (2 %) with * = 1540 m (dedicated short runs). Measurement of elastic scattering in the range 10-3 < |t| < 10 GeV2

An extensive CMS/TOTEM Physics Programme

Conclusions

The experiment at LHC

Backup Slides

Planar technology + CTS(Current Terminating

Structure)

50 µ

m

I2I1

+-

biasing ring Al

p+

n+

cut edge

current terminating

ring

Al

SiO2

n-type bulkp+

50µmMicro-strip Si detectors designed to reduce the inefficiency at the edge.Inefficient edge ~ 50 m

Si CTS edgeless detectors

Proton detection down to 10beam, + d (beam, = 80 600 m)To minimize d : detectors with highly reduced inactive edge (”edgeless”)

Parameters = 2 m(standard step in LHC start-up)

= 90 m(early TOTEM optics)

= 1540 m(final TOTEM optics)

Crossing angle 0.0 0.0 0.0

N of bunches 156 156 43

N of part./bunch (4 – 9) x 1010 (4 – 9) x 1010 3 x 1010

Emittance n [m ∙ rad] 3.75 3.75 1

10 y beam width at RP220 [mm] ~ 3 6.25 0.8

Luminosity [cm-2 s-1] (2 – 11) x 1031 (5 – 25) x 1029 1.6 x 1028

* = 90 m ideal for early running:• fits well into the LHC start-up running scenario;• uses standard injection (b* = 11m) easier to commission than 1540 m optics• wide beam ideal for training the RP operation (less sensitive to alignment)

* = 90 m optics proposal submitted to the LHCC and well received.

Optics and Beam Parameters

Level-1 Trigger Schemes

p

p

p

p

pT1/T2

RP RPCMS

Elastic Trigger:

Single Diffractive Trigger:

Double Diffractive Trigger:

Central Diffractive Trigger:

Minimum Bias Trigger:

(L = 1.6 x 1028 cm-2 s-1)

Extrapolation to the Optical Point (t = 0) at b*=90m

∫ L dt = 2 nb-1

(5 hours @ 1029 cm-2 s-1)

(extrapol. - model) / model in d/dt |t=0 Statistical extrapolation uncertainty

Common bias due to beam divergence : – 2 % (angular spread flattens dN/dt distribution)Spread between most of the models: ±1%Systematic error due to uncertainty of optical functions: ± 3%Different parameterizations for extrapolation tested (e.g. const. B, linear continuation of B(t)): negligible impact

Diffractive scattering is a unique laboratory of confinement & QCD: A hard scale + protons which remain intact in the scattering process Forward protons observed, independent of their momentum losses

Soft Hard Diffractive Scattering large b small b

Valence quarks in a bag with r ~ 0.2fm

Baryonic charge r~0.5fm

Rapidity gap survival & ”underlying” event structures are intimately connected with a geometrical view of the scattering - eikonal approach !

Lorentz contracted longitudinal view

h

p

p

jet

jet

Elastic soft SD

hard SD

hard DPE

ln s

rap

idity

Cross sections are largeMeasure (M,,t)

10mb?29mb? 1mb?

Hard processes:Jet momenta correlated with the initial parton momenta.

Soft processes:Coherentinteractions-impact parameterpicture.

+

‘wee’ parton cloud - grows logarithmically

b b

soft DPE

Try to reach the Coulomb region and measure interference:

• move the detectors closer to the beam than 10 + 0.5 mm• run at lower energy @ √s < 8 TeV

Possibilities of r measurement

asymptotic behaviour:µ1 / ln s for s pred. ~ 0.13 at LHC

sss tot

tot lnd

d

2

29/18

L = (*)1/2

sin((s))

Idea:

Ly large Lx=0

vy = 0

y(220) = /2 x(220) =

(parallel-to-point focussing on y)

(m)

hit distribution (elastic)x = Lxx

* + vxx*

+D

y = Lyy* + vyy *

Optical Functions (* = 90 m)

v = (/*)1/2

cos((s))

Optical functions:

- L (effective length)

- v (magnification)

defined by (betatron function)

and (phase advance);

- D (machine dispersion)

describe the explicit path of particles through the magnetic elements as a function of the particle parameters at IP

= p/p

(x*, y*): vertex position at IP

(x*,y

*): emission angle at IP

t = tx + ty

ti ~ -(pi*)2

Forward Physics: VHE Cosmic Ray Connection

Interpreting cosmic ray data dependson hadronic simulation programs. Forward region poorly known/constr.Models differ by factor 2 or more. Need forward particle/energy measurements e.g. dN/d, dE/d…

Total multiplicity in T2 (5<<7)

p-p collisions @ LHC as predicted by generators tipically used to model hadronic showers generated by VHE CR

Machine Induced Background T1/T2 Detectors: beam-gas interactions: prel. ext. ~ 14 Hz per beam;

~ 19 KHz for MB events (MB = 80 mb, L = 2.4 ·1029 cm-2 s-1)

reduced by vertex reconstruction muon halo (expected to be very small, not yet quantified)

Roman Pot Detectors: beam halo (protons out of design orbit): ext. (* = 1540m) ~ 12·10-4/bunch

reduced by requiring coincidence between RP arms beam-gas interactions: ext. (* = 1540m) ~ 3·10-4/bunch after cuts

reduced with cuts on track angles and multiplicities p-p collision (at IP) background: ext (* = 1540m) ~ (0.4 2)·10-4/bunch

after cuts

reduced with cuts on track angles and hit multiplicities

Tot. elast. evts ~ 3 KHz (L = 1029 cm-2 s-1); prel. expt. S/B ~ (0.6 0.7)·103

Expected Radiation Dose in CMS/TOTEM

In 1 year @

Linst = 1034 cm-2s-1

At RPs locations =>